Firefighters pick through the rubble of a burned out home as they look for clues to a fire that authorities say killed six people in Newark. Photograph: Mel Evans/AP

A fast-moving fire ripped through a single-family home in New Jersey's largest city early Sunday, killing six people, authorities said.

The blaze broke out at the three-story residence at around 4am in Newark, the Essex County Prosecutor's office said. The fire soon spread to another residence, and both homes were destroyed.

The fire doesn't appear to be suspicious, but the cause remains under investigation, said Thomas Fennelly, chief assistant prosecutor. Everyone in the second home managed to escape safely, he said.

Some of the victims were likely related, authorities said.

All that remained of the home Sunday afternoon was the charred frame, with piles of blackened, twisted furniture and belongings spilling out of the empty sills that once held windows. The white fence around the front of the property was still intact.

Carol Valentine sorted through smoke-damaged photographs and photo albums with charred pages on the sidewalk in front of her fiance's home, which is next door to the residence where the fire started. Her fiance had been out of town and returned early Sunday to his home, which was already ablaze.

"He'd been delayed on planes for two days. He had gone to a graduation," she said. "Had he not been delayed, he would have been sleeping, and he probably wouldn't be alive."

She said she didn't know much about the neighbors.

A small storefront church called Tree of Life Ministries, on the other side of the home where six died, appeared undamaged. Neighbors gathered outside the home – many in their Sunday church clothes – shaking their heads at the loss of so many lives.

The Reverend Thomas Ellis lives in the neighborhood and stopped by to offer his support.

"For the city of Newark, this is a sad and tragic day," he said. "The community is hurting."